1. Field of the Invention
The present invention relates to a camshaft driving mechanism for an internal combustion engine, in particular, to a camshaft driving mechanism for an overhead camshaft V-type internal combustion engine whose left and right banks of a cylinder block are set at an angle to each other, including a horizontally opposed engine whose left and right banks are arranged 180.degree..
2. Prior Arts
With respect to a camshaft driving mechanism for a V-type engine or a horizontally opposed engine of which cylinder blocks are set to each other at an angle, since camshafts supported with the cylinder head is rotated by the driving force of the crank shaft so as to open and close the intake and exhaust valves at a specified timing, so many driving mechanisms for rotating the camshafts have ever been proposed.
Japanese patent application laid-open No. Toku-Kai-Hei 2-211307, as shown in FIG. 7, discloses a camshaft driving mechanism for a DOHC engine comprising a crankshaft 100, intermediate shafts 114, 116 provided at each of left and right banks 114, 116 respectively, camshafts 108, a first driving chain 102 which interlocks the crankshaft 100 with the intermediate shafts 114, 116, and second driving chains 118, 120 provided at each of left and right banks 110, 112 respectively which interlocks the intermediate shafts 114, 116 with the cam shafts 108, so as to drive these camshafts with the crankshaft through the intermediate shafts.
Further, Japanese patent application laid-open No. Toku-Kai-Hei 5-156903, as shown in FIG. 8, also discloses a camshaft driving mechanism for a DOH internal combustion engine comprising a crankshaft 100, a camshaft 108 of a right cylinder head 106, an intermediate shaft 114 provided on the left bank side, a first driving chain 102 wound around the crankshaft 100, the camshaft 108 of the right cylinder head 106 and the intermediate shaft 114 so as to drive the camshafts on the right bank side, camshafts 108 of the left bank side and a second driving chain 122 wound around the intermediate shaft 114 and the camshafts 108 on the left bank side so as to drive the camshafts on the left bank side.
However, as shown in the forgoing examples of prior arts, in case of employing chains as a means for transmitting driving force, commonly there is a problem of lengthwise elongation in chains due to wearing, its own elasticity or thermal expansion.
In the former prior art disclosed in Toku-Kai-Hei 2-211307, when the first driving chain 102 is elongated, the rotational angular deviation of the camshaft 108 on the right bank side becomes larger than that of the camshaft 108 on the left bank side with respect to the rotational angle of the crankshaft 100, because when viewing from the tight side of the driving chain the length of the chain between the crankshaft 100 and the intermediate shaft 116 is larger than that between the crankshaft 100 and the intermediate shaft 114.
Similarly, in the latter prior art in Toku-Kai-Hei 5-156903, when the first chain 102 is elongated, the rotational angular deviation of the camshaft 108 on the right bank side becomes larger than that of the camshaft 108 on the left bank side with respect to the rotational angle of the crankshaft 100, because the length (mn+qr+st) on the tight side of the driving chain between the crankshaft 100 and the camshaft 108 on the right bank side is larger than the length (mn+op) on the tight side of the driving chain between the crankshaft 100 and the camshaft 108 on the left bank side.
This deviation in the rotational angle of the camshaft between the left and right banks 110, 112 produces a difference in the valve timing between the left and right banks, this leading to irregular combustion in cylinders or other unfavorable phenomenon.
Further, generally, in the internal combustion engine, accessories such as alternator (generator), water pump and oil pump are driven by a V-belt interconnected with at least one pulley. In most cases, the pulley (crank pulley) is connected with the crank shaft. Generally, the V-belt driving system must be incorporated separately from the chain drive system because lubrication is needed for all components such as chains and sprockets. Therefore, as shown in FIG. 9, a crank pulley 124 for driving accessories is disposed outside of a chain cover 128. Further, the first driving chain 102 must be arranged on a different plane from the second chain 122, therefore the engine 99 requires a substantial lengthwise space for accommodating these driving chains. Since a further lengthwise space is needed for the crank pulley 124, the overall lengthwise size of the engine inevitably becomes large in the prior arts.